Radio communication method, radio communication system, and radio transmission apparatus

ABSTRACT

A transmission apparatus according to the present invention includes: an encoding section; a modulation section; a variable-SF spreading section that performs spreading with variable SF according to control information A from a control section; an IDFT section that performs IDFT on output from the spreading section; a GI inserting section; a parallel/serial conversion section that converts output from the GI inserting section into a serial data sequence; a digital/analog conversion section; and an RF section that transmits a signal from an antenna after converting the signal to a frequency band for analog signal transmission and controlling it to an appropriate transmission power. The control section is configured to generate control information A for determining the variable SF and input the information A to the variable-SF spreading section and the RF section. In control information A, SF is varied depending on transmission power required.

TECHNICAL FIELD

The present invention relates to a radio communication method and aradio transmission apparatus, and more particularly, to a radiocommunication method, a radio communication system, and a radiotransmission apparatus for use in Orthogonal Frequency DivisionMultiplexing (OFDM) and Code Division Multiplexing (CDM) communicationin which data communication is performed with a combination ofmulti-carrier signals typified by CDM-OFDM which transmits spread datausing orthogonal codes.

BACKGROUND ART

In recent years, next-generation mobile communication systems have beenactively studied and a one-frequency repeating cellular system in whichindividual cells use the same frequency band has been proposed as ascheme for improving efficiency of frequency utilization in suchsystems.

For downlink (i.e., communication from a base station apparatus to amobile station), Orthogonal Frequency Division Multiple Access (OFDMA)is the most promising candidate. OFDMA is a system that performscommunication by allocating slots, which are units of access consistingof time axis and frequency axis, among multiple mobile terminalapparatuses and using OFDM signals which are produced by modulatinginformation data such as by 64-ary Quadrature Amplitude Modulation (64QAM) or Binary Phase Shift Keying (BPSK). Because it uses OFDM signals,Peak to Average Power Ratio or PAPR can become very high. High peakpower is not a very large problem in downlink communication which has arelatively large margin in transmission power amplification capability,but can be a significant issue in uplink (i.e., communication from amobile station to the base station apparatus) which has no margin intransmission power amplification capability.

Also, to reduce effect of interference occurring in one-cell repetition,a CDM-OFDM scheme has been studied that distributes one piece of dataacross multiple carriers (this process is called “spreading”) and sendsmultiple pieces of data multiplexed (i.e., spreads data using orthogonalcodes for multiplexing). It is said that in generation of CDM-OFDMsignals, use of orthogonal codes enables data to be demultiplexed,ideally on a receiver, and use of scrambling codes specific to the basestation can spread interference as well.

Non-Patent Document 1 listed below discloses CDM-OFDM (referred to as“OFCDM” in the document). According to the document, in an isolatedcell, better throughput characteristics are obtained when spreading infrequency domain is not used because such a cell is little affected byinterference, whereas in an environment largely affected byinterference, e.g., in a one-frequency repeating system, spreading infrequency domain results in better characteristics.

Meanwhile, Patent Document 1 shown below proposes a scheme for switchingan access method. The scheme shown in the document switches betweenmulti-carrier and single-carrier. The scheme is basically intended foruse in uplink and proposes that single-carrier scheme be selected whenhigh transmission power is required and the OFDM scheme when lowtransmission power is required.

Non-Patent Document 1: IEICE TRANS. COMMUN., VOL. E86-B, NO. 1 JANUARY2003 “Variable spreading-factor-Orthogonal Frequency and Code DivisionMultiplexing (VSF-OFCDM) for Broadband Packet Wireless Access”

Patent Document 1: JP Patent Publication (Kokai) No. 2007-151059 ADISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

Non-Patent Document 1 proposes a scheme to change spreading factor in aCDM-OFDM system in consideration of communication environment. However,because the scheme does not take into account PAPR characteristics ofsignals, it still leaves a problem in expansion of coverage area.

Patent Document 1 describes that effect of PAPR characteristics isreduced by switching the access method between single-carrier andmulti-carrier, but the technique has the problem of wide variations incommunication characteristics because it uses switching between twomethods.

In single-carrier communication, as a communication band becomes wider,characteristics degrade due to effect of frequency selectivity fading.Accordingly, in a communication system using a wide frequency band,degradation in characteristic is significant and throughput decreases.In other words, in downlink communication in a cellular system, coveragearea could be expanded in terms of PAPR characteristics butcharacteristic degradation would affect a wide area.

The present invention has been made in view of such problems, and anobject thereof is to provide a technique for expanding coverage area inthe CDM-OFDM scheme in consideration of PAPR characteristics and withminimized degradation in characteristics.

Means for Solving the Problems

According to an aspect of the present invention, there is provided atransmission apparatus for use in CDM-OFDM transmission in which data istransmitted with a combination of a CDM scheme and an OFDM scheme, thetransmission apparatus comprising: a variable-gain amplifier foradjusting transmission power: a variable-spreading-factor spreadingsection for spreading transmission data across a plurality ofsub-carriers in accordance with a spreading factor using a rotationorthogonal code; and a control section for controlling an amplificationfactor of the variable-gain amplifier and the spreading factor of thevariable-spreading-factor spreading section. Preferably, the controlsection performs control to increase the spreading factor as theamplification factor is increased. To increase the spreading factormeans to use more sub-carriers for transmitting one piece of data. Thespreading factor is defined as SF herein. For a terminal that requireshigh transmission power, communication can be performed withoutdistortion of signals by using a rotation orthogonal code andtransmitting data with a large SF.

Furthermore, there is provided a transmission apparatus for use inCDM-OFDMA transmission in which data is transmitted with a combinationof a CDM scheme and an OFDMA scheme which is composed of a plurality oftime/frequency sub-channels, the transmission apparatus comprising: avariable-gain amplifier for adjusting transmission power: avariable-spreading-factor spreading section for spreading transmissiondata across a plurality of sub-carriers in accordance with a spreadingfactor using a rotation orthogonal code, for each frequency sub-channel;and a control section for controlling an amplification factor of thevariable-gain amplifier and the spreading factor of thevariable-spreading-factor spreading section. Preferably, the controlsection performs control to increase the spreading factor as theamplification factor is increased. Also, the control section preferablyperforms control so that the same spreading factor is used on frequencysub-channels that are on the same time sub-channel which is formed bydividing time into a plurality of time channels. However, it is notstrictly required that the spreading factor be the same, and spreadingfactors may be controlled in the direction that brings them closer tothe same one.

Furthermore, the transmission apparatus preferably comprises a datainserting section for inserting zero data for unused sub-carriers inaccordance with a number by which sub-carriers used are decreased; and asub-carrier allocation section for selecting sub-carriers used forcommunication of spread data and allocating the data to thesub-carriers. By decreasing sub-carriers that are used and selectingsub-carriers with low interference, effect of interference can bereduced. Also, the position of sub-carriers used may be changed.

The sub-carrier allocation section preferably allocates sub-carriers foreach one of groups that are formed by dividing spread data. Thesub-carrier allocation section preferably determines sub-carriers foruse in accordance with quality of individual sub-carriers. By selectingand allocating sub-carriers of good quality to each group, sub-carriersof good quality are more likely to be selected and correlation ofinterference from other cells can be lowered.

The variable-spreading-factor spreading section preferably comprisesdiscrete Fourier transform processing and realizes spreading with avariable spreading-factor by selecting output of one of discrete Fouriertransform processing stages in accordance with the spreading factor.

There is also provided a transmission apparatus for use in data CDM-OFDMtransmission in which data is transmitted with a combination of a CDMscheme and an OFDM scheme, the transmission apparatus comprising: avariable-spreading-factor spreading section for spreading data infrequency domain using a rotation orthogonal code; and an RF section forchanging output power in accordance with the spreading factor.Preferably, the variable-spreading-factor spreading section comprisesdiscrete Fourier transform processing and realizes spreading with avariable spreading-factor by selecting output of one of discrete Fouriertransform processing stages in accordance with the spreading factor.

According to another aspect of the invention, there is provided a radiocommunication system composed of a base station and a plurality ofterminal apparatuses that are connected to the base station, wherein atransmission method by the transmission apparatus according to any oneof the above-described aspects is used for communication from the basestation to the terminal apparatuses, and DFT-s-OFDM communication isused for communication from the terminal apparatuses to the basestation. Preferably, a first terminal apparatus among the plurality ofterminal apparatuses has a function of transmitting and receiving datato and from the base station and a second terminal apparatus that isdifferent from the first terminal apparatus. Preferably, a terminalapparatus among the plurality of terminal apparatuses that is connectedto the base station has a function of receiving data transmitted fromthe base station using a rotation orthogonal code and transmitting thereceived data to another terminal apparatus using the DFT-s-OFDMcommunication scheme.

According to another aspect of the invention, there is provided aCDM-OFDM transmission method for transmitting data with a combination ofa CDM scheme and an OFDM scheme, the method comprising a step of using arotation orthogonal code as a spreading code for use in frequencyspreading in the CDM scheme and changing a spreading factor inaccordance with transmission power.

There is also provided a CDM-OFDMA transmission method for transmittingdata with a combination of a CDM scheme and an OFDMA scheme which iscomposed of a plurality of time/frequency sub-channels, the methodcomprising a step of using a rotation orthogonal code as a spreadingcode for use in frequency spreading in the CDM scheme and changing aspreading factor for each frequency sub-channel in accordance withtransmission power.

In the transmission methods, a method that is implemented by any of theconfigurations specified for the transmission apparatuses above can beused.

Further, there may be also provided a CDM-OFDMA transmission method fortransmitting data with a combination of a CDM scheme and an OFDMA schemewhich is composed of a plurality of time/frequency sub-channels, themethod comprising: a step of using a rotation orthogonal code as aspreading code for use in frequency spreading in the CDM scheme andsetting a spreading factor in accordance with transmission power foreach frequency sub-channel; and a step of changing the number ofsub-carriers used, or a CDM-OFDM transmission method for transmittingdata with a combination of a CDM scheme and an OFDM scheme, the methodcomprising: an amplification step of adjusting transmission power; and avariable-spreading-factor spreading step of spreading transmission dataacross a plurality of sub-carriers in accordance with a spreading factorusing a rotation orthogonal code, wherein an amplification factor usedin the amplification step and the spreading factor used in thevariable-spreading-factor spreading step are controlled, or a CDM-OFDMAtransmission method for transmitting data with a combination of a CDMscheme and an OFDMA scheme which is composed of a plurality oftime/frequency sub-channels, the method comprising: an amplificationstep of adjusting transmission power; an a variable-spreading-factorspreading step of spreading transmission data across a plurality ofsub-carriers in accordance with a spreading factor using a rotationorthogonal code for each frequency sub-channel, wherein an amplificationfactor used in the amplification step and the spreading factor used inthe variable-spreading-factor spreading step are controlled.

A program for causing a microcomputer such as a computer to execute thefunctions of the transmission methods and/or transmission apparatuses isalso encompassed within the scope of the invention, or a recordingmedium having a microcomputer-readable program for executing thatprogram stored thereon is also contemplated.

ADVANTAGES OF THE INVENTION

For a terminal that requires high transmission power, communication canbe performed without distortion of signals by using rotation orthogonalcodes and transmitting data with a high SF. By changing SF in accordancewith required transmission power, more flexible communication can berealized while minimizing degradation in characteristics that resultsfrom communication being performed in a wide frequency band.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows an example of input/output power characteristics of ahigh-output amplifier.

FIG. 1B is a functional block diagram showing an example of atransmission apparatus for use with a communication technique accordingto an embodiment of the present invention.

FIG. 2 is an exemplary functional block diagram showing details ofvariable-SF spreading section 3 of FIG. 1B.

FIG. 3 is a schematic diagram showing exemplary operations of thespreading section shown in FIG. 2.

FIG. 4 shows PAPR characteristics for various SFs with a total of 64sub-carriers.

FIG. 5 shows an exemplary configuration of the variable-SF spreadingsection with a reduced circuit scale.

FIG. 6 shows an exemplary block configuration of a receiver.

FIG. 7 illustrates a case that uses IDFT in the variable-SF inversespreading section 68 shown in FIG. 6.

FIG. 8 is a functional block diagram showing an exemplary configurationof a transmission apparatus according to a second embodiment of theinvention.

FIG. 9 is a functional block diagram showing an exemplary configurationof a reception apparatus according to the second embodiment.

FIG. 10 is a functional block diagram showing an exemplary configurationof a transmission apparatus according to the second embodiment.

FIG. 11 shows examples of allocation to consecutive sub-carriers andallocation to groups of four sub-carriers.

FIG. 12 illustrates PAPR characteristics obtained with various numbersof sub-carriers for grouping when the total number of sub-carriers is 64as in the previous examples and 32 sub-carriers are used.

FIG. 13 is a functional block diagram showing an exemplary configurationof a reception apparatus according to the second embodiment.

FIG. 14 shows an example of a frame format for use in the secondembodiment.

FIG. 15 shows an example of cell layout for communication in which eachcell is divided into three areas, A, B, and C.

FIG. 16 shows variations in transmission power in individual sectorswith time interference coordination effected.

FIG. 17 is a functional block diagram showing an exemplary configurationof a DFT-s-OFDM transmitter.

FIG. 18 shows an example of a frame format for use when relay isperformed.

DESCRIPTION OF SYMBOLS

1 . . . encoding section, 2 . . . modulation section, 3 . . .variable-SF spreading section, 4 . . . IDFT section, 5 . . . guardinterval (GI) inserting section, 6 . . . parallel/serial (P/S)conversion section, 7 . . . digital/analog (D/A) conversion section, 8 .. . RF section, 10 . . . control section, 85 . . . variable-gainamplifier, 86 . . . high output amplifier.

BEST MODE FOR CARRYING OUT THE INVENTION

A communication technique according to embodiments of the invention willbe described below by taking MC-CDM (Multi-Carrier CDM) as an example ofa Code Division Multiplexing (CDM) multi-carrier communication schemewith a variable spreading-factor (hereinafter spreading factor will bereferred to as “SF”). As OFDM (Orthogonal Frequency DivisionMultiplexing) is used as an example of the multi-carrier scheme, it maysometimes be referred to as CDM-OFDM herein. Descriptions herein assumethat there are 64 sub-carriers for OFDM signals in total, and SF of 1(i.e., normal OFDM), 4, 16, and 64 can be used. SF indicates how manysub-carriers are used for sending one piece of data. Also, theembodiments described below show cases where the same number of piecesof data as SF are multiplexed (i.e., the same number of orthogonal codesas SF are used) unless otherwise specified. Further, the embodimentsenvisage downlink communication, which generally refers to communicationfrom a base station to a mobile station, unless otherwise specified.

A communication technique according to a first embodiment of theinvention will be now described with reference to drawings. The firstembodiment of the invention is premised on services in a local area. Forservices in a local area, effect of interference is not a significantconsideration. The Non-Patent Document 1 shown above also describes thateffect of interference is little in such an area and use of OFDM signalswith SF set to 1, that is, normal OFDM signals, is the way to obtain thehighest throughput. As a factor that allows SF to be set to one, it ispremised that linearity of a high-output amplifier of the base stationcan be kept high and that OFDM signals with high PAPR (Peak to AveragePower Ratio) can he transmitted without distortion. Distortion ofsignals caused by an amplifier will be described with reference to FIG.1A.

FIG. 1A shows an example of input/output power characteristics of ahigh-output amplifier. In the figure, the horizontal axis representsinput power and the vertical axis represents output power, and the boldarrows show variations in instantaneous input power of a signal. Anamplifier has high linearity when average input power to the amplifieris set to I1 in FIG. 1A (hereinafter such a region will be called a“linear region”), for example. In other words, a constant gain isobtained with any instantaneous input power of an input signal. On theother hand, when the average input power to the amplifier is set to I2,a constant gain cannot be obtained with respect to variations ininstantaneous input power (i.e., gain is less when instantaneous inputpower is high than when it is low). This leads to problems like signaldistortion or leakage of signals outside a band (hereinafter, such aregion will be called a “non-linear region”).

Conventional discussions on communication have assumed that signals arenot distorted, that is to say, processing is linear, on a base stationas shown in the Non-Patent Document 1. However, for expansion ofcoverage area per base station, it is required to further increasetransmission power, and due to limitation on improvement of amplifierperformance, operation in the non-linear region has to be considered. Toenable operation in the non-linear region, it is important to limitsignal PAPR, i.e., the variation shown by the bold arrows in FIG. 1A, toa small range. A normal OFDM signal scheme uses a signal with thisbold-arrow range being wide, known to be a scheme that does not lenditself to non-linear operation of an amplifier.

FIG. 1B is a functional block diagram showing an example of atransmission apparatus for use with the communication techniqueaccording to an embodiment of the invention. In the figure, referencenumeral 1 denotes an encoding section for encoding data, and 2 denotes amodulation section for performing modulation, such as BPSK (modulationperformed in this section may be sometimes called “primary modulation”).Reference numeral 3 denotes a variable-SF spreading section forperforming spreading with a variable SF based on control information Afrom a control section 10. The variable-SF spreading section will bedescribed in more detail later. Reference numeral 4 denotes an IDFTsection for performing Inverse Discrete Fourier Transform (IDFT) onoutput from the spreading section. Needless to say, in general, InverseFast Fourier Transform (IFFT) is often used. Reference numeral 5 denotesa Guard Interval (GI) inserting section. GI is used in OFDM signals inmost existing systems for reducing effect of inter-symbol interference.Reference numeral 6 denotes a parallel/serial (P/S) conversion sectionfor converting output from the GI inserting section 5 into a serial datasequence. Reference numeral 7 denotes a digital/analog (D/A) conversionsection for converting digital data into analog data. Reference numeral8 denotes an RF section for transmitting a signal from an antenna afterconverting the signal to a frequency band for analog signal transmissionand controlling the signal to an appropriate transmission power. The RFsection 8 includes a variable-gain amplifier 85 for adjusting outputpower and a high-output amplifier 86, which was mentioned above.Finally, reference numeral 10 denotes a control section, configured togenerate control information A for determining variable SF and input thecontrol information A to the variable-SF spreading section 3 and the RFsection 8. In control information A, SF is changed depending ontransmission power required.

FIG. 2 is an exemplary functional block diagram showing details of thevariable-SF spreading section 3 of FIG. 1B. In the figure, referencenumerals 31 to 33 denote first to third serial/parallel conversionsections (S/P conversion sections) for converting input data from serialto parallel; reference numerals 34 to 36 denote first to third spreadingsections for spreading data; reference numerals 37 and 38 denote datajoining sections for joining pieces of data together; and referencenumeral 39 denotes a data selection section for selecting SF accordingto control signal A. The first, second, and third S/P conversionsections 31, 32, and 33 have different rates, outputting 64, 16, and 4pieces of parallel data, respectively. In the first to third spreadingsections 34 to 36, spreading takes place in accordance with theirrespective SFs. Operations of the first to third data spreading sections34 to 36 will be described later taking the spreading section 34 as anexample.

As 64 sub-carriers is assumed herein, the data joining sections 37 and38 connect multiple outputs from the second and third spreading sections35 and 36 to produce data for 64 sub-carriers. As an output of thesecond spreading sections 35 is 16 pieces of data, the data joiningsection 37 joins four outputs to generate data equivalent to 64sub-carriers, and as an output of the third spreading section 36 is 4pieces of data, the data joining section 38 connects 16 outputs togenerate data equivalent to 64 sub-carriers. Output of the S/Pconversion section 31 may sometimes be input to the data selectionsection 39 as it is. This is the case when an OFDM signal with SF of 1,i.e., a normal OFDM signal, is generated. The data selection section 39selects and outputs data for use in accordance with control signal A. InFIG. 2, numbers (4, 16, and 64) indicated on arrows showing signal flowrepresent the number of pieces of data that are input or output at atime.

FIG. 3 is a schematic diagram illustrating exemplary operations of aspreading section shown in FIG. 2. Input data Dk is complex data from D1through D64, represented by Dk=ak+bkj (k is data number). Here, ak andbk are values that depend on the primary modulation scheme; for example,when the primary modulation scheme is BPSK, ak=±1 and bk=0. Also,j×j=−1.

The present embodiment employs rotation orthogonal codes as codes forCDM. A rotation orthogonal code Cm of a code length of L (m is asubscript indicative of code type, being a natural number from 1 to L)can be represented as: Cm=(e^(j×θ×(m−1)×0), e^(j×θ×(m−1)×1), . . . ,e^(j×θ×(m−1)×(L−1))), where θ=2×π/L.

FIG. 3 shows rotation orthogonal codes for L=64.Input Dk is multipliedby individual elements constituting Cm. For the sake of simplicity,however, pairs that make k=m are used as pairs for multiplication.Thereby, Dk is split into 64 pieces of data. The 64 results are addedfor each element to produce output of the spreading section. When thisoutput signal is denoted as CM, Cm=(Σm (am+bmj)×e^(j×θ×(m−1)×0),Σm(am+bmj)×e_(j×θ×(m−1)×1), . . . , Σm (am+bmj)×e^(j×θ×(m−1)×(L−1))),where Σm is addition regarding m.

While FIG. 3 shows an example of 64-spreading, the spreading sections 35and 36 of FIG. 2 perform spreading with L=16 and L=4, respectively. Thevariable-SF spreading section then selects a signal corresponding to theSF specified in the control signal A.

To demonstrate the advantage of using rotation orthogonal signals forspreading, FIG. 4 shows PAPR characteristics for various SFs when atotal of 64 sub-carriers are used. In the figure, the horizontal axisrepresents PAPR (dB) and the vertical axis represents the probability(%) that a signal has a PAPR exceeding the PAPR indicated on thehorizontal axis. However, this data represents values calculated throughsimulation of patterns of some signals. Also, numbers preceding CDM-OFDMindicate SF, and characteristics denoted by square plot points in thefigure represent PAPR characteristics with SF=1 (i.e., normal OFDM),triangle points with SF=4, circle points with SF=16,and “x” points withSF=64.

As can be seen from the plot of FIG. 4, PAPR characteristics areimproved as SF increases. By way of example, if input power to thehigh-output amplifier is set with PAPR characteristics of 1% (i.e., at“1” on the vertical axis of the plot) as a reference, a margin of about0.3 dB, 1.3 dB, and 3 dB are produced for SF of 4, 16, and 64,respectively, relative to the normal OFDM signal. In other words, whenthe same high-output amplifier is used, the average input power can beincreased by 0.3 dB, 1.3 dB, and 3 dB for SF of 4, 16, and 64,respectively, and output power can accordingly be increased as well.

Describing this more specifically, when “Iin” denotes the limit ofaverage input power to the high-output amplifier during transmission ofnormal OFDM signals (a level of electric power, input of an averageinput power exceeding which makes effect of non-linear distortionnon-negligible), the limit of average input power can be increased up to(Iin+0.3) dB for SF4, (Iin+1.3) dB for SF16, and (Iin+3) dB for SF64.

As described above, when considering communication in a local area andexpansion of coverage area, for a terminal that requires hightransmission power, communication can be performed without distortion ofsignals by using rotation orthogonal codes and transmitting data with ahigh SF. In addition, since PAPR characteristics are dependent on SF, bychanging SF in accordance with required transmission power, moreflexible communication can be realized while minimizing degradation incharacteristics that result from communication being performed in a widefrequency band.

FIG. 5 shows an exemplary configuration of the variable-SF spreadingsection of a reduced circuit scale. This configuration implementsspreading by DFT and can be realized with several butterfly operationunits and memory. DFT permits high-speed computation and also cansimplify circuitry as compared to the configuration shown above. WhileFIG. 5 shows time thinning method, this configuration can be alsorealized by frequency thinning method.

FIG. 5 shows butterfly operation units with a radix of 4. Although thefigure shows that 16×3 (stages)=48 butterfly operation units (denoted asDFT4s) are necessary for the sake of simplifying the followingdescription, as these butterfly operation units have exactly the samestructure, this configuration could be realized with at least onebutterfly operation unit. DFT processing has a notion of processingstages corresponding to radix, and 64 (the number of DFT points) isrepresented as 4 (i.e., radix)³. Thus, there are three stages ofprocessing in this embodiment.

When its inputs are denoted as X1, X2, X3, X4 and outputs are as Y1, Y2,Y3, Y4, radix-4 butterfly operation is represented by Equation (1):

[Equation 1]

Y1=X1+X2+X3+X4

Y2=X1−jX2−X3+jX4

Y3=X1−X2+X3−X4

Y4=X1+jX2−X3−jX4   (2)

Here, j represents a complex number. The DFT4s in FIG. 5 are alloperations represented by Equation (1). Also, on arrows indicative ofdata flow between DFT4s, signals are shown as “Wa”. This “Wa” is anumerical value called a twiddle factor to be multiplied to data flowingon each arrow. As the number of DFT points is 64 in the presentembodiment, the twiddle factor is defined by Equation (2):

[Equation 2]

Wa=e _(j×2π×α/64)   (2)

The variable-SF spreading section shown in FIG. 5 includes the S/Pconversion section 31, a data rearrangement section 50, the dataselection section 39, and the butterfly operation sections (the DFT4s).In the figure, blocks having the same function as those in thefunctional block diagram of FIG. 2 are given the same referencenumerals.

Data input to the variable-SF spreading section of FIG. 5 is subjectedto S/P conversion. FIG. 5 shows a case where 64 pieces of parallel data,which is equal to the number of the maximum sub-carriers, are generated.The S/P output is input to the data selection section 64, and selectionof this output means selection of SF=1,namely the OFDM scheme.

Then, the pieces of parallel-converted data are rearranged. Here, thedata are rearranged in accordance with SF specified in control signal A.FIG. 5 shows the result of rearrangement with a SF of 64 selected. WhenSF is 16, rearranged outputs will be: D1, D5, D9, D13, D2, D6 . . . ,and when SF is 4, rearranged outputs will be: D1, D2, D3, D4, D5, D6 . ..

When input in the sequences shown here, data are put in a neat order.When SF is 16, D1 through D16 undergo spreading in the first 16sub-carriers, and D17 through D32 in the next 16 sub-carriers, and dataare subsequently spread in order of their subscripts. This is similarwhen SF is 4. When this order of data is not necessary, rearrangement isnot necessarily required if data order is known between the transmitterand the receiver.

When SF is 64, processing at all of the stages is performed and outputof stage 3 is selected in the data selection section 64. When SF of 16is selected, the data selection section 64 selects output of stage 2.When SF is 4, the data selection section 64 selects output of stage 1.

Thus, by selecting output of one of stages in one run of DFT, it ispossible to generate a CDM signal with variable SF using rotationorthogonal codes.

Now, an exemplary configuration of a receiver is shown. Note that theconfiguration shown here employs IDFT corresponding to DFT of FIG. 5 asvariable-SF inverse spread processing. FIG. 6 shows an exemplary blockconfiguration of a receiver. In the figure, reference numeral 61 denotesan RF section that has the capability of converting a received signal toa frequency band in which the signal can be converted into a digitalsignal; 62 denotes an A/D conversion section for converting an analogsignal to a digital signal; 63 denotes a symbol synchronization sectionfor achieving symbol synchronization for an OFDM signal and removing GIwhich is added on the sending side; 64 denotes a S/P conversion sectionfor performing serial to parallel conversion in order to conform asymbol-synchronized signal to the number of DFT input points (convertsto 64 points in the present embodiment); and 65 denotes a DFT sectionfor performing DFT processing. Needless to say, in general, FFT (FastFourier Transform) is often used. Of signals resulting from DFT, one foruse in channel estimation is input to a channel estimation section 66,in which a channel estimation process is performed. A signal for data isinput to a channel compensation section 67, in which channelcompensation is conducted on the signal with respect to the channelestimation signal. Here, processing that is less prone to noiseenhancement is preferably performed.

After undergoing channel compensation, the signal is input to thevariable-SF inverse spreading section 68 and subjected to a variable-SFinverse spreading process therein. To the variable-SF inverse spreadingsection 68, control information A′ regarding SF is input from thecontrol section 74, and a variable-SF inverse spreading process isperformed on the basis of SF. The inverse spreading process will bedescribed in more detail later. After variable SF inverse spreading, thesignal is converted to serial data in the parallel/serial conversionsection 69. Then, in a demodulation section 70, the signal undergoesdemodulation corresponding to the primary modulation applied in thetransmission apparatus to be input to a decoding section 71. Thedecoding section 71 performs processing corresponding to encodingapplied in the transmission apparatus to obtain transmitted data. It isalso assumed that the control section 74 has the capability of supplyingSF information and SF is already known at the time of data demodulation.

FIG. 7 illustrates a case that uses IDFT in the variable-SF inversespreading section 68 of FIG. 6. This configuration is based on IDFT asin spreading, and IDFT can be realized with several butterfly operationunits and memory. Since IDFT permits high-speed arithmetic processing,circuitry can be simplified as compared to conventional inversespreading. While FIG. 7 illustrates frequency thinning method as anexample, the configuration may also be realized by time thinning method.

FIG. 7 shows butterfly operation units with a radix of 4. While thefigure shows that there are 16 butterfly operation units (denoted asIDFT4s in FIG. 7) per stage, that is to say, 16×3 (stages)=48 butterflyoperation units for the sake of simplifying the following descriptions,as these butterfly operation units are of exactly the same structure,this configuration could be realized with at least one butterflyoperation unit.

When its outputs are denoted as X1, X2, X3, X4 and inputs are as Y1, Y2,Y3, Y4, radix-4 butterfly operation is represented by Equation 3 below:

[Equation 3]

X1=(Y1+Y2+Y3+Y4)/4

X2=(Y1+jY2−Y3−jY4)/4

X3=(Y1−Y2+Y3−Y4)/4

X4=(Y1−jY−Y3+jY4)/4   (3)

Equation (3) is a solution of Equation (1) for X1, X2, X3 and X4. TheIDFT4s in FIG. 7 are all operations represented by Equation (3). Also,on arrows indicating data flow between IDFT4s, signals are shown as Wa(“a” being a number). This “Wa” is a numerical value called a twiddlefactor, to be multiplied to data flowing on each arrow. As there are 64IDFT points in the present embodiment, the twiddle factor is defined byEquation (4):

[Equation 4]

Wa=e ^(−j×2π×α/64)   (4)

The variable-SF inverse spreading section 68 of FIG. 7 is composed ofthe data rearrangement section 80, the data selection section 79, andbutterfly operation sections (IDFT4s).

Data input to the variable-SF inverse spreading section 68 of FIG. 7 isinput to the data selection section 79, and selection of this data meansselection of SF 1, namely the OFDM scheme. Then, IDFT processing instage 1 is performed. Output of this stage corresponds to output, thatis, an inverse spread value, for SF=4. Similarly, output of stage 2corresponds to the inverse spread value for SF=16, and output of stage 3to that for SF=64. The data rearrangement section 80 rearranges data.This is because it is necessary to rearrange data in accordance with SF,as, when Rk denotes received data, stage 1 outputs data in ascendingorder of R1, R2, R3, R4, F5 . . . , stage 2 outputs in the order of R1,R5, R9, R13, R2, R6 . . . , and stage 3 outputs in the order of R1, R17,R33, R49, R2, R18 . . . , which is also shown in the figure. This isbased on the assumption that rearrangement is also performed on thetransmission apparatus so that data is in the same positionalrelationship.

A receiver configuration having such a variable inverse spreading unithas the advantage of facilitating inverse spreading according to SF(Spreading Factor).

The present embodiment envisages downlink communication and a featurethereof is that it provides some steps between the OFDM and thesingle-carrier scheme. And it describes an exemplary circuitconfiguration that can realize the steps through the use of relationshipbetween DFT (Discrete Fourier Transform) and SF (Spreading Factor).

Now, a second embodiment of the invention will be described withreference to drawings. The first embodiment described above is intendedfor use with services in a local area, which do not have to takeinterference into account, whereas the second embodiment envisages aone-frequency repeating system that will be used for communication overa pubic network in future. A one-frequency system is a system in whichbase stations constituting cells perform communication all using thesame frequency band, and accordingly a significant issue thereof issecuring of communication quality at a cell edge.

The aforementioned Non-Patent Document 1 discloses a method for reducingeffect from other cells by performing spreading and multiplying ascrambling code in a cellular system. Spreading and scrambling areimplemented in the frequency domain of OFDM signals. As describedearlier, the Non-Patent Document 1 does not take PAPR into account.Nonetheless, the document shows that communication efficiency isimproved by setting data efficiency (=multiplex number/SF) to about 0.25and using scrambling so as to enable interference to be spread throughinverse spreading on a receiver.

Since the first embodiment of the present invention performs spreadingin frequency domain but sets data efficiency to 1, it cannot spreadeffect when an interference signal has a similar signal form, whichleads to degradation in communication quality. This is becausecorrelation becomes high due to use of the same spreading code in allcells that give rise to effect of interference. When consideringinterference alone, multiplication of a scrambling code is an option,which is also shown in the Non-Patent Document 1, but it significantlydegrade PAPR characteristics of signals. The present embodiment shows aconfiguration of transmission and reception apparatuses that reduceseffect of interference without degrading PAPR characteristics.

FIG. 8 is a functional block diagram showing an exemplary configurationof a transmission apparatus according to the second embodiment of theinvention. In the figure, blocks having the same functions as thoseshown in FIG. 1 are given the same reference numerals. The transmissionapparatus shown in FIG. 8 further includes a repetition section 11 and ascrambling section 12 in addition to the configuration of FIG. 1B. Fromthe control section 10, control information B is input to the repetitionsection 11. The repetition section 11 has the capability of repeating aninput signal a number of times specified in control signal B. Thescrambling section 12 has the capability of scrambling input data with arandom code. The scrambling capability can be realized by multiplying aninput signal by ±1 randomly in a simplest case. A random signal forscrambling can be of any of various patterns, but is preferably one thathas no correlation with neighboring base stations. Also, to avoidstationary errors, it is preferable to change the random signal for eachpacket or frame.

The data repetition section 11 and the scrambling section 12 enableinterference to be spread on a receiver. In addition, by making thenumber of repetitions large, tolerance to interference is improved. Thedata repetition and scrambling do not change the data pattern of theprimary modulation essentially. Consequently, PAPR characteristicsbecome dependent on a SF specified in control information A, and settingSF to 64 can keep PAPR characteristics in a good condition (i.e.,characteristics of 64CDM-OFDM in FIG. 4).

The data repetition section 11 can also multiplex data. For example,while the example above showed simple repetition by a designated numberof times, odd-numbered pieces of data may be multiplied by an orthogonalcode that is different from the one for even-numbered pieces of data andadded before being scrambled. Such a method has an advantage in thatloss of data rate resulting from repetition can be prevented to someextent, though PAPR characteristics are slightly degraded.

FIG. 9 is a functional block diagram showing an exemplary configurationof a reception apparatus according to the present embodiment. In thefigure, blocks having the same function as those shown in FIG. 6 aregiven the same reference numerals. This configuration further includes adescrambling section 75 and an addition section 76 in addition to theconfiguration of FIG. 6. Control information B′ is input to the additionsection 76 from the control section 74. The descrambling section 75 hasthe capability of descrambling input data with a random code. The randomcode can be realized by dividing the data by the random code used by theother end of communication. The addition section 76 performs addition ofdata according to control information B′. The number of pieces of datato be added is the same as the number of repetitions used on the sendingside.

The descrambling and addition on the receiver can spread interference,providing the advantage of enabling highly accurate communication evenin an area with high interference, such as at a cell edge.

While FIGS. 8 and 9 are described given an area with high interference,in an area with little interference, such as in the center of a cell, ifrepetition and scrambling on the sending side as well as descramblingand addition on the receiving side are not performed, the situationwould correspond to the configuration shown in the first embodiment andthe effects thereof could be obtained. In practice, there will be noproblem if the number of repetitions is set to one, and whetherscrambling is performed or not does not have a significant influence.

Next, a method for avoiding interference through processing in frequencydomain will be shown. While the previous example spreads interference byrepeating time domain data (i.e., output of the modulation section 2),the following example reduces interference by reducing the number ofsub-carriers used and using sub-carriers at different positions.

FIG. 10 is a functional block diagram showing an exemplary configurationof a transmission apparatus according to the present embodiment. In thefigure, blocks having the same function as those in the functional blockdiagram of FIG. 1 are given the same reference numerals. Theconfiguration further includes a zero-inserting section 13 and asub-carrier allocation section 14 in addition to the configuration ofFIG. 1. From the control section 10, control information C is input tothe zero-inserting section 13 and control information D is to thesub-carrier allocation section 14.

Hereinafter, signal flow with a reduced number of sub-carriers used willbe described. The zero-inserting section 13 inserts zeros in accordancewith a number by which sub-carriers used are decreased. By way ofexample, as the present embodiment is described given 64 sub-carriers,when 16 sub-carriers are used, following input of 16 pieces of data fromthe modulation section 2, 48 pieces of “0” data are inserted in thezero-inserting section 13.

Control signal A is also changed in accordance with the number ofsub-carriers used. When 16 sub-carriers are used, control signal Aspecifies a SF of 16 (i.e., makes the number of sub-carriers used equalto SF). With such signal processing, output of the variable-SF spreadingsection 3 consists of a sequence of 16 pieces of data followed by 48consecutive zeros.

Because the present embodiment reduces the number of sub-carriers used,when the same electric power is used as when all the sub-carriers areused in terms of the entire band, transmission power per sub-carrier canbe increased and hence effect of interference can be reduced.

In this case, characteristics can be further improved if the sub-carrierallocation section 14 selects sixteen consecutive sub-carriers having aslittle interference as possible and allocates those sub-carriers tooutput of the variable-SF spreading section 3. The allocation here isinput to the sub-carrier allocation section 14 as control information Dfrom the control section 10.

Effect of interference can be reduced by decreasing the number ofsub-carriers used and selecting sub-carriers with low interference, butcorrelation still remains in interference because the situation wherethe same code is used for spreading as in other cells stays unchanged.Thus, a method for further reducing correlation of interference fromother cells will be shown below.

This method is similar to the previous example up to insertion of zeros.Then, the sub-carrier allocation section 14 groups input data other thanzeros. For example, as 16 pieces of input data have signals in theprevious example, the 16 pieces of data are placed into four groups eachhaving four pieces of data. Then, a sub-carrier with good quality isselected and allocated to each of the four groups. By making such a wayof allocation, it is possible to increase the possibility thatsub-carriers of good quality can be selected and to lower correlation ofinterference from other cells. This is because of data rearrangementperformed at the time of inverse spreading on the receiver, which lowerscorrelation with rotation orthogonal codes used in other cells.

FIG. 11 shows examples of allocation to consecutive sub-carriers andallocation to groups of four sub-carriers. In FIG. 11, the horizontalaxis represents frequency and rectangles each represent one sub-carrier.Shaded squares represent sub-carriers that actually have signal powerand plain squares represent sub-carriers to which no signal power isallocated.

FIG. 11( a) shows output from the variable-SF spreading section 3, whichis also input to the sub-carrier allocation section 14. With control bythe variable-SF spreading section 3, signal power concentrates in lowerranges in the frequency domain (i.e., the hatched area). The sub-carrierallocation section 14 performs allocation in accordance with controlinformation D from the control section 10. FIG. 11( b) shows allocationto all consecutive 16 sub-carriers, and FIG. 11( c) shows allocation togroups of four sub-carriers.

By grouping fewer sub-carriers, it is possible to increase thepossibility that sub-carriers of good quality can be selected as well asthe effect of reducing the influence of interference, but PAPRcharacteristics call degrade more significantly.

FIG. 12 illustrates PAPR characteristics obtained with various numbersof sub-carriers that are grouped when the total number of sub-carriersis 64 as in the previous examples and 32 sub-carriers are used. Thevertical and horizontal axes represent the same as in FIG. 4. The value“1” in S1 in FIG. 12 indicates the number of sub-carriers to be grouped.This value being 1 means sub-carriers is not grouped. OFDM representsPAPR characteristics of an OFDM signal with 32 sub-carriers, and32CDM-OFDM represents characteristics obtained when 32 sub-carriers areconsecutively arranged. With such grouping, although there isdegradation in PAPR characteristics, when four sub-carriers are grouped(S4), degradation in PAPR characteristics is as small as about 1.5 dB bycomparison even at PAPR characteristics of 1% (i.e., comparison at “1”on the vertical axis of the plot).

It thus proves to have the effect of power reduction and hence issufficiently applicable to communication to a cell edge.

FIG. 13 is a functional block diagram showing an exemplary configurationof a reception apparatus according to the present embodiment. In thefigure, blocks having the same function as those shown in FIG. 9 aregiven the same reference numerals. This configuration further includesan extraction section 77 and a zero-deleting section 78 in addition tothe configuration of FIG. 9. From the control section 74, controlinformation C′ is input to the zero-deleting section 78 and controlinformation D′ is to the extraction section 77.

Hereinafter, signal flow of when a reduced number of sub-carriers areused will be described.

The extraction section 77 extracts sub-carriers to which power has beenallocated on the transmission apparatus in accordance with controlinformation D′. This means extraction of signals on the shadedsub-carriers shown in FIG. 11( b) or 11(c). The extraction section 77further has the capability to input the extracted signals to the IDFTsection (variable-SF inverse spreading section) 68 such that the signalsgather in lower ranges in IDFT input, meaning that signals are input asillustrated in FIG. 11( a). The variable-SF inverse spreading section 68sets SF in accordance with the number of sub-carriers being used.Thereafter, the zero-deleting section 78 deletes irrelevant data. Thus,these sections have the reverse functions to those of the transmissionapparatus.

As described above, the present embodiment can lower interferencecorrelation associated with use of the same spreading code byrearranging, on the receiver, sub-carriers to which signals have beenallocated. It therefore has the advantage of enabling highly accuratecommunication even in an area with high interference, such as at a celledge.

Now, a communication technique according to a third embodiment of theinvention will be described with reference to drawings. The thirdembodiment of the invention is an example of application of the CDM-OFDMsignal shown in the first or second embodiment to an OFDMA system. Whilethe first and second embodiments showed an OFDM system with 64sub-carriers, the present embodiment forms an OFDMA system regarding theOFDM system as one frequency sub-channel. Such a system is called hereina CDM-OFDMA system. The present embodiment shows a case with 12sub-channels, in which case there are 64×12=768 sub-carriers in total.

FIG. 14 shows an example of a frame format for use in the presentembodiment. In the figure, the vertical axis represents frequency andthe horizontal axis represents time. On the frequency axis, there arefrequency sub-channels from F1 to F12, and on the time axis, there aretime sub-channels from T1 to T9. Although space is present betweensub-channels, it does not mean that sub-carriers or time that is notactually used is necessary. In FIG. 14, the twelve frequencysub-channels F1 through F12 and nine time sub-channels T1 through T9constitute one frame, by repeating which, a base station and a terminalstation communicate with each other. When a unit composed of frequencysub-channels and time sub-channels is called a slot, slots are allocatedwhen communication is performed and the base station and the terminalstation communicate in those slots. Also, a time sub-channel made up ofT1 and F1 through F12 (the shaded slot in FIG. 14) is a frame in whichinformation regarding the structure of the frame or the like istransmitted as control information and which needs to send data to theentire cell.

FIG. 15 shows an example of cell layout for communication in which eachcell is divided into three areas, A, B, and C. The areas A to C formedby such division of a cell are called sectors, and all the sectors usethe same frequency. In FIG. 15, however, areas are arranged such thatthe same area is not specified in neighboring sectors. When interferenceis taken into consideration in this situation, an area represented by acertain symbol will be affected most by signals from areas representedby the other symbols than that symbol. That is to say, sector A isaffected most by sectors B and C.

In such a cell layout, when in sector A, for example, a user wants tocommunicate with a terminal that is positioned at a cell edge, it isdesirable that signal power, that is, interference power, of sectors Band C is small. By positioning signals so as to form such a condition,interference can be reduced. Hereinafter, a method in which each sectoror cell considers transmission power of other cells or sectors so thateach other's interference power is minimized will be called interferencecoordination.

FIG. 16 shows variations in transmission power in individual sectorswith time interference coordination effected. As the plots are based onthe frame format shown in FIG. 14, time sub-channels available for datacommunication are eight time sub-channels T2 to T9. FIG. 16 shows anexample in which each sector has two, four, and two time sub-channelsfor the maximum, minimum, and medium transmission power, respectively,and the transmission power of the sector is controlled to be minimizedwhile data is transmitted at the maximum transmission power in othersectors. By thus implementing interference coordination, interferencebetween sectors can be reduced. Interference coordination can also berealized by using frequency sub-channels. However, interferencecoordination is desirably performed using time sub-channels in thepresent embodiment because the embodiment uses rotation orthogonal codesas orthogonal codes and considers reduction of PAPR in a CDM-OFDMsystem.

In FIG. 16, in time sub-channels in which data is transmitted at themaximum transmission power (e.g., T2 and T3 of sector A), SF is set to64 in all frequency sub-channels. Thereby, PAPR degradation can becontrolled. This because excellent PAPR characteristics are obtainedwhen SF is 64 in CDM-OFDM signals as shown in the first embodiment.Typically, PAPR characteristics of OFDM signals degrade as the number ofsub-carriers increases. Therefore, PAPR degradation can be prevented bycarrying out interference coordination on time sub-channels andperforming spreading with the same SF on individual frequencysub-channels, as shown in this embodiment.

In FIG. 16, SF is set to 16 for time sub-channels on which data istransmitted at a medium transmission power and to 1 for timesub-channels on which data is transmitted at the minimum transmissionpower. As to T1 frame, it is premised that control information isreceived by all terminals as mentioned above. Therefore, it is notpreferable to lower its transmission power. It is not preferable eitherto change time sub-channels targeted for interference coordination(i.e., not desirable to change the position of time sub-channels amongsectors) when considering handover between sectors. Therefore, themethod of reducing the sub-carriers used and spreading interference byway of arrangement of sub-carriers used should be adopted as shown inthe second embodiment.

Next, a communication technique according to a fourth embodiment of theinvention will be described with reference to drawings. Its circuitconfiguration using DFT, which was shown in the first embodiment, isalmost the same as that for the DFT-s-OFDM (DFT-spread-OFDM)communication scheme. FIG. 17 is a functional block diagram showing anexemplary configuration of a DFT-s-OFDM transmitter. In the figure,blocks having the same function as those of the transmitter of FIG. 2are given the same reference numerals. In FIG. 17, reference numeral 80denotes a DFT section for performing time-frequency conversion, and 81denotes a sub-carrier allocation section that selects and allocatessub-carriers. The DFT-s-OFDM communication scheme has good PAPRcharacteristics as a modulation scheme for single carrier and has beenproposed for use as the communication scheme for uplink. As can beunderstood from the configuration, it is the same configuration as theCDM-OFDM system using rotation orthogonal codes shown in the firstembodiment if SF is made equal to the number of sub-carriers. However,as this embodiment is intended for use in uplink, it envisagesmultiplexing with other terminals through FDM (Frequency DivisionMultiplexing). Therefore, the sub-carrier allocation section 81 isinserted as a block that selects sub-carriers for use in transmission.

Consequently, the reception apparatus shown in the first embodimentwould be able to demodulate signals generated in the DFT-s-OFDMcommunication scheme if constraints such as timing permit. It thereforemeans that if DFT-s-OFDM is used as the communication scheme for uplinkin a system that uses the transmission scheme shown in the firstembodiment (i.e., CDM-OFDM using rotation orthogonal codes) in downlink,a terminal capable of connecting to a base station is enabled tocommunicate with other terminals as well, meaning that retransmission bymobile terminals can be easily realized.

As a specific example, downlink communication between the base stationand terminal station B in which terminal A relays data will be describedgiven the OFDMA system shown in the third embodiment. FIG. 18 shows anexample of a frame format for use when relay is performed, where slotsF1 to F3 at T3 (the cross-hatched slots) are slots allocated tocommunication from the base station to terminal A and slots T7 to T9 atF1 (the gray slots) are slots allocated to communication from terminal Ato terminal B. It is assumed here that the base station does not performtransmission in slots T7 to T9 at F1 .

From the base station to terminal A, data destined for terminal B istransmitted. For this transmission, there is no particular SF that isoptimal, and any scheme may be used as long as terminal A can receivethe data. However, to complete retransmission within the same frame,transmission of the data preferably finishes at an early stage in theframe (i.e., a stage with a smaller time sub-channel number). Therefore,the present embodiment completes the transmission at T3 using multiplesub-channels as mentioned above. The terminal A demodulates the receiveddata, and transmits the data to terminal B using the communicationscheme for uplink, namely DFT-s-OFDM (the transmitter shown in FIG. 17).In general, high-output amplifiers used in terminals are often inferiorin performance to those of base stations. Thus, to minimize degradationin PAPR characteristics, it is preferable to a use fewer sub-carriers.The present embodiment therefore uses sub-carriers equivalent to onesub-channel of downlink (64 sub-carriers) to transmit DFT-s-OFDM signals

Accordingly, the base station allocates the slots T7 to T9 of F1 tocommunication from terminal A to terminal B. Terminal B demodulates datasetting SF to 64 with the reception scheme for downlink. However, whenFDD (Frequency Division Duplex) is used in uplink and downlink, it isnecessary to change RF frequency to the downlink frequency duringcommunication from terminal A to terminal B.

As described, in a cellular system or the like, by using CDM-OFDM thatemploys rotation orthogonal codes as the communication scheme for onedirection and DFT-s-OFDM as that for the other direction, retransmissionby terminals becomes possible without having to further prepare ademodulation circuit and the like.

INDUSTRIAL APPLICABILITY

The present invention is applicable to communication apparatuses.

1-18. (canceled)
 19. A transmission apparatus for use in CDM-OFDMAtransmission in which data is transmitted with a combination of a CDMscheme and an OFDMA scheme which is composed of a plurality oftime/frequency sub-channels, the transmission apparatus comprising: avariable-gain amplifier for adjusting transmission power: avariable-spreading-factor spreading section for spreading transmissiondata across a plurality of sub-carriers in accordance with a spreadingfactor using a rotation orthogonal code, for each frequency sub-channel;and a control section for controlling an amplification factor of thevariable-gain amplifier and the spreading factor of thevariable-spreading-factor spreading section.
 20. The transmissionapparatus according to claim 19, wherein the control section performscontrol to increase the spreading factor as the amplification factor isincreased.
 21. The transmission apparatus according to claim 19, whereinthe control section performs control so that a same spreading factor isused on frequency sub-channels that are on the same time sub-channelwhich is formed by dividing time into a plurality of time channels. 22.The transmission apparatus according to claim 19, further comprising: adata inserting section for inserting zero data for unused sub-carriersin accordance with a number by which sub-carriers used are decreased;and a sub-carrier allocation section for selecting sub-carriers used forcommunication of spread data and allocating the data to thesub-carriers.
 23. The transmission apparatus according to claim 22,wherein the sub-carrier allocation section allocates sub-carriers foreach one of groups that are formed by dividing spread data.
 24. Thetransmission apparatus according to claim 22, wherein the sub-carrierallocation section determines sub-carriers for use in accordance withquality of individual sub-carriers.
 25. The transmission apparatusaccording to claim 19, wherein the variable-spreading-factor spreadingsection comprises discrete Fourier transform processing, and realizesspreading with a variable spreading-factor by selecting output of one ofdiscrete Fourier transform processing stages in accordance with thespreading factor.
 26. A CDM-OFDMA transmission method for transmittingdata with a combination of a CDM scheme and an OFDMA scheme which iscomposed of a plurality of time/frequency sub-channels, the methodcomprising: a step of using a rotation orthogonal code as a spreadingcode for use in frequency spreading in the CDM scheme and setting aspreading factor in accordance with transmission power for eachfrequency sub-channel.
 27. The CDM-OFDMA transmission method accordingto claim 26, further comprising; a step of changing the number ofsub-carriers used.